Peter M. Hoffmann, Ph.D.

DETROIT—Identifying water's behavior when confined at the molecular level has been, in the field of nanofluidics, a source of controversy - one that a Wayne State University researcher and his colleagues may have put to an end.

Peter M. Hoffmann, Ph.D., associate professor of physics and materials science at WSU's College of Liberal Arts and Sciences, has found that at the nanoscale, liquid water transforms into a rubber-like solid when squeezed at a certain rate. The study is featured in the Oct. 15, 2010 issue of Nature India and the Sept. 2010 issue of Physical Review Letters with a special Viewpoint written by well-known researchers from University of Illinois. Only 100 out of 18,000 papers in this prestigious journal are selected for a Viewpoint review each year. The study has shed new light on the nanofluidics debate over the nature of confined water's mechanical properties.

Water, which makes up nearly 70 percent of the human body, is nanoconfined between proteins that make up the cell's organelles.

"Usually the water in our cells is considered as a rather static bystander," said Hoffmann. "But water is the most important liquid in the universe because it is the one essential ingredient we need to support life. Knowing how water behaves in tiny channels and tiny spaces is important for the design of future devices that would, for example, probe arterial blood and continually measure blood sugar or other markers."

Hoffmann explored how water reacts when its molecules are gently squeezed at speeds "so slow it would take a few months to just cover a distance of one foot," he said. Yet the impact of this speed, as Hoffmann and his colleagues have proved, alters water's behavior drastically.

"When we squeezed water at a speed of 0.8 nanometers per second and beyond until the tip reached the surface, the water suddenly changed from a viscous honey-like liquid to an almost solid-like material that reacted elastically, like rubber," said Hoffmann. A sensitive atomic force microscope (AFM) that was built by his team made these precise nanoscale measurements possible.

Hoffmann and his team also saw that water spontaneously orders into layers, each as thin as a single water molecule, when confined. To reach this conclusion, Hoffmann constricted water against a flat surface with the tiny AFM tip until the space between the two shrank to a width of only a few nanometers.

Although the research is fundamental, Hoffmann said, "the discoveries may play a role in how cellular components move and transmit forces, as well as aid in the design of nanomechanical devices."

Others who participated in the study are Shah Khan, graduate student in WSU's physics department who performed the measurements; and George Matei, Ph.D., former WSU graduate student who built the AFM used in the study along with Shivprasad Patil, Ph.D., former postdoctoral fellow at WSU and current professor of physics at the Indian Institute of Science Education and Research (IISER) in Pune, India.

To view the Physical Review Letters Viewpoint, visit http://physics.aps.org/articles/v3/73. To view the Nature India feature, visit http://www.nature.com/nindia/2010/101015/full/nindia.2010.143.html.

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